1. Field of the Invention
This invention relates generally to circuits and methods for programming memory array structures. More particularly, this invention relates circuits and methods for boosting a gate voltage of a select switching transistor of spin-torque magnetic random access memory (MRAM) cells for programming spin-torque magnetic random access memory (MRAM) cells in an array.
2. Description of Related Art
The term Spin-RAM or spin-torque MRAM refers to a spin torque transfer magnetization switching (STS) MRAM. In this context, the term “spin” refers to the angular momentum of electrons passing through an MTJ that will alter the magnetic moment of a free layer of an MTJ device. Electrons possess both electric charge and angular momentum (or spin). It is known in the art that a current of spin-polarized electrons can change the magnetic orientation of a free ferromagnetic layer of an MTJ via an exchange of spin angular momentum. The major difference between conventional MRAM and spin-torque MRAM is their programming mechanism. Programming is accomplished by bi-direction currents passing through the Spin-RAM magnetic layers, while conventional MRAM is programmed by fields generated by external current or currents. Reading is similar for both types of the memories, namely detecting the resistance difference of magnetic tunnel junction depending on whether the free layer magnetic polarity is parallel or anti-parallel with the fixed layer. The programming currents required for spin-torque MRAM is much lower than the conventional MRAM and allows a more scalable with geometry reduction.
“A Novel Nonvolatile Memory with Spin Torque Transfer Magnetization Switching: Spin-Ram”, Hosomi, et al., IEEE International Electron Devices Meeting, 2005. IEDM Technical Digest. December 2005, pp.: 459-462, provides a nonvolatile memory utilizing spin torque transfer magnetization switching (STS), abbreviated Spin-RAM. The Spin-RAM is programmed by magnetization reversal through an interaction of a spin momentum-torque-transferred current and a magnetic moment of memory layers in magnetic tunnel junctions (MTJs), and therefore an external magnetic field is unnecessary as that for a conventional MRAM.
Refer now to FIG. 1 for a block diagram illustrating a spin-torque MRAM cell and its peripheral circuitry of the prior art. A spin-torque MRAM cell 100 consists of an MTJ element 105 and a select switching Metal Oxide Semiconductor (MOS) transistor 110. The MTJ element 105 is composed of a pinned ferromagnetic layer 102 and a free ferromagnetic layer 104, and a tunnel barrier layer 103. The drain of the MOS transistor 110 is connected through a nonmagnetic layer to the pinned ferromagnetic layer 102. The free ferromagnetic layer 104 is connected to a bit line 115 and the source of the MOS transistor 110 is connected the source select line 120. The bit line 115 and source select line 120 are connected to the bipolar write pulse/read bias generator 125. The bipolar write pulse/read bias generator 125 provides the necessary programming current to the MTJ element 105 through the bit line 115 and the source select line 120. The direction being determined by logic state being programmed to the MTJ element 105.
The gate of the select switching MOS transistor 110 is connected to a word line 130. The word line 130 transfers a word line select voltage to the gate of the select switching MOS transistor 110 to activate the select switching MOS transistor 110 for reading or writing the logic state of the MTJ element 105. A sense amplifier 135 has one input terminal connected to the bit line and a second input terminal connected to a voltage reference circuit 140. When the word line 115 has the word line select voltage activated to turn on the select switching MOS transistor 110, the bipolar write pulse/read bias generator 125 generates a bias current that passes through MTJ element 105. A voltage is developed across the MTJ element 105 that is sensed by the sense amplifier 135 and compared with the reference voltage generator to determine the logic state written to the MTJ element 105. This logic state is transferred to the output terminal of the sense amplifier 135 as to the data output signal 145.
Refer to FIG. 2 for a description of an array of spin-torque MRAM cells 100. The spin-torque MRAM cells 100 are arranged in rows and columns. The gate of the MOS transistor of each of the spin-torque MRAM cell 100 on a row of the spin-torque MRAM cells 100 is connected to a word line 200a, 200b, . . . , 200n−1, 200n to receive the word line select signal to activate the MOS transistor for writing and reading of a selected spin-torque MRAM cell 100. One terminal of each of MTJ element of the spin-torque MRAM cells 100 on a column of spin-torque MRAM cells 100 is connected to a bit line 205a, 205b, . . . , 205m. The source of each MOS transistor of each of the spin-torque MRAM cells 100 is connected to a source select line 210a, 210b, . . . , 210m. During a write operation, a programming voltage is transferred either from a selected bit line 205a, 205b, . . . , 205m through the selected spin-torque MRAM cell 100 to the selected source select line 210a, 210b, . . . , 210m or from selected source select line 210a, 210b, . . . , 210m through the selected spin-torque MRAM cell 100 to the a selected bit line 205a, 205b, 205m, dependent upon the logic state to be written to the selected spin-torque MRAM cell 100.
U.S. Patent Application 2007/0279968 (Luo, et al.) provides a magnetic memory magnetic memory cells attached to local word lines that are in turn connected to global word lines. The magnetic memory further includes bit lines connected to the magnetic elements, and source lines connected to the selection device. Each magnetic memory cell includes a magnetic element and a selection device connected with the magnetic element. The magnetic element is programmed by first and second write currents driven through the magnetic element in first and second directions. The local word lines are connected with the selection device of and have a first resistivity. Each global word line corresponds to a portion of the local word lines and has a resistivity lower than the first resistivity.
U.S. Pat. No. 6,816,405 (Lu, et al.) describes a segmented word line architecture for cross point magnetic RAM. The MRAM includes a plurality of magnetic memory cells, a plurality of local word lines. Each of the local word lines being operatively coupled to at least one memory cell for assisting in writing a logical state of the at least one memory cell. Each of the local word lines are connected to a global word lines. The global word lines are isolated from the memory cells and are coupled by write circuits to the global word lines. Bit lines are operatively coupled to the memory cells for selectively writing a logical state of one or more of the memory cells. Each of the write circuits is configurable as a current source and/or a current sink for supplying and/or returning, respectively, at least a portion of a write current for assisting in writing one or more memory cells. The write circuits are configured to selectively distribute the write current across the global word lines so that stray magnetic field interaction between selected memory cells and half-selected and/or unselected memory cells is reduced.
U.S. Pat. No. 7,345,945 (Jeon, et al.) teaches a line driver circuit for a semiconductor memory device. The semiconductor memory device has a word line driver circuit configured in stages. A plurality of sub word line driver circuits are connected, in parallel, to each main word line, and provide a sub word line enable signal to a selected sub word line in response to a main word line enable signal provided through a main word line. A plurality of (local) word line driver circuits are connected in parallel, to each sub word line and provide a local word line enable signal to a selected local word line in response to the (main/sub) word line enable signal so as to operate a plurality of memory cells connected to the selected local word line.
U.S. Pat. No. 6,424,563 (Honigschmid) describes an MRAM memory cell that includes a magnetoresistive resistor and a switching transistor. The magnetoresistive resistor is located between a central metallization plane and an upper metallization plane. The central metallization plane serves for the word line stitch and also for writing. A word line boost circuit is provided in the stitch region of each cell, with the result that the critical voltage is not achieved in the magnetoresistive resistor but the switching transistor can be turned on.